You're so confident there's no way she would be able to resist telling you about the man of her dreams. [ Rin's found a new source of fun: convincing this guy to get wailed on by Ciel. ] In that case, you should go ask her right now.

There's no sense in waiting. It'd do no good and the holiday only lasts for the entire day unless she's the type to celebrate it traditionally... even so, she'll need to be gifted first.